A recent approach for gasoline engines is to operate them in a lean-burn mode, where the A/F ratio is higher than stoichiometry, in order to improve fuel economy. Traditional three-way catalysts for treating engine exhaust gases operate efficiently to convert nitrogen oxides (NOx), carbon monoxide and hydrocarbons under stoichiometric conditions but are not efficient to convert the NOx in lean-burn situations. Lean-NOx catalysts formulated to convert NOx in an oxidizing exhaust are generally based on zeolites and convert the NOx by using reducing agents like hydrocarbons and carbon monoxide in the exhaust gas. They suffer from lack of good high temperature hydrothermal durability, that is, at high temperatures the structure of the zeolite collapses and dealumination occurs. Another approach for lean-burn exhaust gas treatment has focused on the use of lean-burn NOx traps, i.e., materials which are able to absorb nitrogen oxides from the exhaust gas during lean-burn operation and then later release them when the oxygen concentration in the exhaust gas is reduced. For example, when the A/F ratio is made rich or stoichiometric. Conventional NOx absorbents are alkaline earth metals like barium with a precious metal catalyst like platinum carried on alumina. The widely held mechanism for this absorption phenomena is that during lean-burn operation the platinum first oxidizes NO to NO.sub.2 and the NO.sub.2 subsequently forms a nitrate complex with the trapping material, e.g., the barium. In the regeneration mode, under a stoichiometric or rich environment, the nitrate decomposes and the NOx released is reduced catalytically over the platinum with reducing species like HC or CO in the exhaust gas.
One serious deficiency of such conventional absorbent materials is that the barium reacts with sulfur oxides generally present in exhaust gas to form barium sulfate. This inactivates the barium for NOx absorption. It is suggested that to decompose the barium sulfate it should be subjected to elevated temperatures of at least 600.degree. C. or more in reducing gas conditions. One negative aspect of this regeneration process is that it leads to detrimental changes in the NOx absorbent such as reduced surface area and crystallization of the aluminate phases thereby reducing the efficiency of the NOx absorbent. Alkali metals like potassium have also been suggested as NOx absorbents, however, they are even more easily deactivated by sulfur than alkaline earth metals like barium. Repeated regeneration of the absorbent by heating, as discussed above, contributes to a loss of surface area in the alumina support material and contributes toward further sintering in the platinum precious metal responsible for the conversion of NOx to NO.sub.2. Precious metal sintering results in a decrease in the active sites that convert NOx to NO.sub.2, and hence a decrease in the total amount of NOx trapped on the available absorbent.
It would be desirable if a NOx absorbent could be found which would be more resistant to sulfur poisoning while being a good NOx absorbent. It would also be very desirable if the improved material could withstand repeated heating to de-sulfur, i.e., reactivate the absorbent, without significantly reducing the amount of surface area.
Sol-gel processing allows for the low temperature preparation of aluminum oxide materials of high purity and controlled microstructure. These materials can be in such forms as gels, aerogels and xerogels. Sol-gel processes have been found useful in preparing supported and unsupported inorganic membranes for chemical sensor and separation applications, for automobile catalyst washcoats, and for optical coatings for automotive glass. For example, in U.S. Pat. No. 5,403,807 by Narula, a method is disclosed for making a single phase metal-alumina sol-gel material which includes alkaline earth metal and optionally lanthanides. These materials are disclosed as being stable at high temperatures making them ideally useful as catalyst supports.
We have now unexpectedly found that alumina matrix materials including alkali metals and/or alkaline earths and optionally lanthanides when made by sol-gel techniques are excellent materials for use as NOx absorbents. These materials overcome the deficiencies of prior art NOx absorbents.
In U.S. application Ser. No. 09/134992 filed Aug. 17, 1998 U.S. Pat. No. 6,129,898 and entitled "NOx Trap Catalyst For Lean Burn Engines" commonly assigned with the present invention, a sol-gel oxide material is disclosed useful for NOx absorption. It comprises oxides of aluminum, magnesium and zirconium.